Blow-by gas passage abnormality detecting system for internal combustion engines

ABSTRACT

A blow-by gas passage abnormality detecting system for an internal combustion engine detects, only under the condition that the running state of the engine is idle, a leakage of a blow-by gas coming from a blow-by gas passage for returning the blow-by gas produced in the engine to an intake passage, on the basis of a parameter varying with a change in the air flow to be sucked into the engine. The abnormality such as leakage or clogging in the blow-by gas passage is detected if the ISC opening of an idle speed control valve measured as a load amount at an idle time of the engine is smaller than a predetermined value. Alternatively, the abnormality may be detected by detecting the blow-by gas pressure in the blow-by gas passage downstream of a PCV valve by a gas pressure sensor.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application is related to and incorporates herein byreference Japanese Patent Applications No. 8-343013 filed on Dec. 24,1996 and No. 8-344776 filed on Dec. 25, 1996.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a blow-by gas passageabnormality detecting system for detecting an abnormality in a blow-bygas passage to recirculate the blow-by gas, as will leak from thecombustion chamber of an internal combustion engine to a crankcase, tothe side of an intake passage.

[0004] 2. Related Art

[0005] There is known a blow-by gas recirculating system for preventingthe blow-by gas from being released into the atmosphere, by providing ablow-by gas passage for the blow-by gas, as will leak from thecombustion chamber of an internal combustion engine to the crankcase, torecirculate it to the side of the intake passage.

[0006] When an abnormality such as breakage in or detachment of theblow-by gas passage occurs, the blow-by gas may be released contrary tothe prevention of environmental pollution of recent years.

SUMMARY OF THE INVENTION

[0007] Thus, the present invention has an object to provide a blow-bygas passage abnormality detecting system for an internal combustionengine, as can minimize the release of the blow-by gas into theatmosphere and the adverse effect on the internal combustion engine bydetecting an abnormality in the blow-by gas passage properly.

[0008] In a blow-by gas passage abnormality detecting system for aninternal combustion engine, according to a first aspect of theinvention, the abnormality of the blow-by gas passage for returning theblow-by gas in the internal combustion engine to the side of the intakepassage is decided on the basis of the amount of load when the runningstate of the internal combustion engine is idle. By thus grasping theload amount at the idling time of the internal combustion engine, theabnormality of the blow-by gas passage can be properly detected tominimize the release of the blow-by gas into the atmosphere and theadverse effect on the internal combustion engine.

[0009] In a blow-by gas passage abnormality detecting system of theinternal combustion engine, according to the second aspect of theinvention, the abnormality in the blow-by gas passage is decided on thebasis of the pressure in the blow-by gas passage. By the simple andinexpensive construction for detecting the pressure in the blow-by gaspassage, the abnormality of the blow-by gas passage can be properlydetected to minimize the release of the blow-by gas into the atmosphereand the adverse effect on the internal combustion engine.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Other objects, features and advantages of the present inventionwill become more apparent from the following detailed description whenread with reference to the accompanying drawings. In the drawings:

[0011]FIG. 1 is a schematic diagram showing the entire construction of ablow-by gas passage abnormality detecting system for an internalcombustion engine according to a first embodiment of the invention;

[0012]FIG. 2 is a flow chart showing a procedure for processing anabnormality decision in an ECU used in the blow-by gas passageabnormality detecting system according to the first embodiment;

[0013]FIG. 3 is a graph showing a change in a decision value used in theroutine of FIG. 2 against an engine RPM in the first embodiment;

[0014]FIG. 4 is a flow chart showing a procedure for processing after anabnormality decision in the ECU used in the blow-by gas passageabnormality detecting system according to the first to third embodimentsof the present invention;

[0015]FIG. 5 is a flow chart showing a procedure for processing anabnormality decision in the ECU used in the blow-by gas passageabnormality detecting system according to the second embodiment of thepresent invention;

[0016]FIG. 6 is a graph showing a change in a decision value used in theroutine of FIG. 5 against an engine RPM in the second embodiment;

[0017]FIG. 7 is a flow chart showing a procedure for processing anabnormality decision in the ECU used in the blow-by gas passageabnormality detecting system according to the third embodiment of theinvention;

[0018]FIG. 8 is a graph showing a change in a decision value of theroutine of FIG. 7 against an engine RPM in the third embodiment;

[0019]FIG. 9 is a flow chart showing a procedure for deciding thenormality/abnormality of an intake line device or the like to be addedto the blow-by gas passage abnormality detecting systems of an internalcombustion engine according to the first to third embodiments of theinvention;

[0020]FIG. 10A is a flow chart showing a procedure for deciding thenormality/abnormality of a combustion system device to be added to theblow-by gas passage abnormality detecting systems of an internalcombustion engine according to the first to third embodiments of theinvention;

[0021]FIG. 10B is a flow chart showing a procedure forallowing/inhibiting diagnoses at an electric load fluctuation to beadded to the blow-by gas passage abnormality detecting systems of aninternal combustion engine according to the first to third embodimentsof the invention;

[0022]FIG. 11 is a schematic diagram showing the entire construction ofa blow-by gas passage abnormality detecting system for an internalcombustion engine according to the fourth embodiment of the invention;

[0023]FIG. 12 is a flow chart showing a procedure for processing anabnormality decision in an ECU used in the blow-by gas passageabnormality detecting system according to the fourth embodiment of theinvention;

[0024]FIG. 13 is an abnormality decision table showing a decision valueof a negative pressure against a blow-by gas pressure level of FIG. 12;

[0025]FIG. 14 is a graph showing changes in the decision value of thenegative pressure of FIG. 13 against the load fluctuation of theinternal combustion engine in the fourth embodiment;

[0026]FIG. 15 is a schematic diagram showing a modification of anarrangement construction of a gas pressure sensor, as used in theblow-by gas passage abnormality detecting system of an internalcombustion engine according to the fourth embodiment of the invention,with respect to a blow-by gas passage;

[0027]FIG. 16 is a flow chart showing a processing procedure after anabnormality decision in an ECU used in the blow-by gas passageabnormality detecting system according to the fourth embodiment of theinvention;

[0028]FIG. 17 is a flow chart showing a procedure for processing anabnormality decision in an ECU used in the blow-by gas passageabnormality detecting system according to the fifth embodiment of theinvention;

[0029]FIG. 18 is an abnormality decision table showing a decision valueagainst the negative pressure difference of FIG. 17 in the fifthembodiment; and

[0030]FIG. 19 is a flow chart showing a procedure for deciding thenormality/abnormality of an intake line device or the like to be addedto the blow-by gas passage abnormality detecting systems of an internalcombustion engine according to the fourth and fifth embodiments of theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031] The invention will be described in detail with reference to itsvarious embodiments.

[0032] <Embodiment 1>

[0033] In an intake system of an internal combustion engine 1, as shownin FIG. 1, the air, as introduced from an air cleaner 11 at an upstreamside, flows through an air flow meter 33, as arranged in an intakepassage 12, a throttle valve 13, as arranged in the intake passage 12,and a surge tank 14 at a downstream side and is mixed with fuel, asinjected into an intake manifold 15 by an injector 16, until the mixtureis introduced from an intake port 17 through an intake valve 18 into acombustion chamber 21 of each engine cylinder. Midway of a bypasspassage 19 connected bypassing the throttle valve 13, on the other hand,there is arranged an ISC (Idle Speed Control) valve 20 for controllingthe engine RPM (rotational speed) at an idle time to a predeterminedidle RPM. In an exhaust system of the internal combustion engine 1, theexhaust gas from the combustion chamber 21 is discharged from an exhaustvalve 22 through an exhaust port 23 and an exhaust passage 24 by thenot-shown three-way catalytic converter, until it is discharged into theatmosphere. Here, the air flow meter 33 detects the flow amount ofintake air passing through the intake passage 12. Upstream of thecatalytic converter in the exhaust passage 24, on the other hand, thereis arranged an oxygen concentration sensor 25 for detecting the oxygen(O₂) concentration in the exhaust gas.

[0034] On the other hand, the blow-by gas, as will leak out of thecombustion chamber 21 through the clearance between a piston 2 and acylinder wall 3 into a crankcase 4, is introduced from the upper portionof the crankcase 4 via a communication passage 5 into a head cover 6 ofthe internal combustion engine 1. The upper portion of the head cover 6is connected through a communication passage 26 to the intake passage 12and through a blow-by gas passage 27 to the surge tank 14 downstream ofthe throttle valve 13. Midway of the blow-by gas passage 27, there isarranged a PCV (Positive Crankcase Ventilation) valve 28 acting as aflow rate control valve having the well-known construction and drivenpneumatically by the negative pressure (or the differential pressure).

[0035] This PCV valve 28 is so opened/closed by the negative pressure inthe surge tank 14 that its air passing area is reduced, when thenegative pressure as the intake pressure is high as at the idle time, toreduce the blow-by gas flow from the head cover 6 into the surge tank14, and that its air passing area is enlarged, when the negativepressure as the intake pressure is enlarged at an acceleration time toincrease the blow-by gas flow from the head cover 6 into the surge tank14.

[0036] That is, as the inside of the surge tank 14 takes a negativepressure, the inside of the head cover 6 is ventilated with the freshair which is introduced out of the intake passage 12 via thecommunication passage 26, so that the blow-by gas in the head cover 6 isreturned, while its flow rate being regulated by the PCV valve 28, viathe blow-by gas passage 27 into the surge tank 14.

[0037] An ECU (Electronic Control Unit) 40 is constructed as a logicaloperation circuit including: the not-shown well-known central processingunit CPU; a ROM storing a control program; a RAM for storing variousdata; a B/U (backup) RAM; an input/output circuit; and bus lines forconnecting the foregoing components. To this ECU 40, there areindividually inputted a signal on the oxygen concentration in theexhaust passage 24 from the oxygen concentration sensor 25, a signal onthe intake air flow to pass through the intake passage 12 from the airflow meter 33, and so on. From the ECU 40, there are individuallyoutputted drive signals to the injector 16 arranged in the intakemanifold 15, the ISC valve 20 arranged in the bypass passage 19, and thelike.

[0038] The ECU 40, more particularly the CPU, is programmed to performprocesses of FIG. 2 showing a procedure for processing an abnormalitydecision in the ECU 40 used in the blow-by gas passage abnormalitydetecting system.

[0039] In FIG. 2, at step S101, it is decided whether or not the runningstate of the internal combustion engine 1 is idle. When the decision ofstep S101 is NO indication that the running state is other than the idletime, the present routine is ended without any operation. On the otherhand, when the decision of step S101 is YES, that is, when the throttleopening of the throttle valve 13 is less than a predetermined value toindicate that the running state is idle, the routine advances to stepS102, at which it is decided whether or not the prevailing ISC openingof the ISC valve 20 is less than a decision value or reference A. Thisdecision value A is set to the ISC opening for the idle time, as cannotoccur if the blow-by gas passage 27 is normal.

[0040] The setting of this decision value A is based on the fact thatthe control duty ratio of the ISC valve 20 is ordinarily 20 to 30%, forexample, when the blow-by gas passage 27 is normal, whereas an unlikelyexcess intake supply is made from the side of the blow-by gas passage27, when this passage 27 causes the leakage or disengagement, so thatthe control duty ratio of the ISC valve 20 drops as low as 0%. Here, thedecision value A may be so set at each engine RPM as to rise accordingto the rise in the engine RPM, as illustrated by the graph of FIG. 3.

[0041] When the decision of step S102 is NO indicating that the ISCopening is no less than the decision value A, the routine advances tostep S103, at which it is ended by determining the normality of theblow-by gas passage 27 from the head cover 6 to the surge tank 14. Onthe other hand, when the decision of step S102 is YES indicating thatthe ISC opening is less than the decision value A, it is determined thatthe blow-by gas passage 27 has some leakage or disengagement, theroutine advances to step S104, at which an abnormality is determined inthe leakage or disengagement of the blow-by gas passage 27 from the headcover 6 to the surge tank 14. Then, the routine advances to step S105,at which it is ended by executing an after-abnormality processing. Thus,it is possible to properly detect the abnormality (i.e., the leakage orthe disengagement) of the blow-by gas passage 27 from the head cover 6to the surge tank 14.

[0042] Step S105, that is a procedure for processing after anabnormality decision in the ECU 40 used in the blow-by gas passageabnormality detecting system of an internal combustion engine accordingto first embodiment is shown in FIG. 4. Here, when it is decided thatthe blow-by gas passage 27 has the abnormality (i.e., the leakage or thedisengagement), the reliabilities in the various detection steps and thecontrol steps, as follows, cannot be kept so that these steps areinhibited.

[0043] In FIG. 4, at first step S201, the trouble detection of the airflow meter 33 for detecting the intake flow in the intake passage 12 isinhibited; at next step S202, the trouble detection of the ISC valve 20is inhibited; at next step S203, the trouble detection of the ISCsystem; at next step S204, the trouble detection of an EGR (Exhaust GasRecirculation) system (although not shown in FIG. 1) is inhibited; andat next step S205, the trouble detection of the fuel system (such as afuel injection time control system) is inhibited.

[0044] The routine advances to step S206, at which a misfire detectionis inhibited; at next step S207, the F/B (i.e., feedback) control of theidle RPM in the ISC system is inhibited (or to lower the gain); at nextstep S208, an A/F (i.e., air/fuel ratio) learning control is inhibited(or to inhibit the main A/F or to lower the gain); at next step S209,the EGR control is inhibited (or to lower the gain); and at next stepS210, the purge control of an evaporated fuel is inhibited (or to lowerthe gain), and the present routine is ended.

[0045] Thus, the blow-by gas passage abnormality detecting system of thepresent embodiment performs deciding the abnormality of the blow-by gaspassage 27 to return the blow-by gas, which is produced in the internalcombustion engine 1 on the basis of the load amount when the runningstate of the internal combustion engine 1 is idle, to the side of theintake passage 12 of the internal combustion engine 1. By taking intoconsideration the load amount when the running state of the internalcombustion engine 1 is idle, therefore, the abnormality of the blow-bygas passage 27 can be properly detected to minimize the release of theblow-by gas into the atmosphere and the adverse effect on the internalcombustion engine 1.

[0046] Moreover, the blow-by gas passage abnormality detecting systemfor an internal combustion engine according to the present embodiment isenabled, by the simple and inexpensive construction for measuring theload amount by using the intake flow amount, to minimize the release ofthe blow-by gas into the atmosphere and the adverse effect on theinternal combustion engine 1.

[0047] Moreover, the blow-by gas passage abnormality detecting systemfor an internal combustion engine according to the present embodimentmeasures the ISC opening as the control amount of the ISC valve 20 forcontrolling the engine RPM at the idle time of the internal combustionengine 1, by the intake air amount, and decides that the blow-by gaspassage 27 has the leakage or disengagement, by the abnormality decisionachieved by the ECU 40, when the ISC opening is smaller than thedecision value A or the predetermined value. As a result, it can bedecided from the more intake flow coming from the others than theordinary one that the control of the ISC value 20 at the idle time ofthe internal combustion engine 1, that is, the ISC opening is smallerthan the predetermined value, and it can be deemed that the leakage ordisengagement occurs in the blow-by gas passage 27. By thus taking intoconsideration the ISC opening of the ISC valve 20 at the idle time, theabnormality of the blow-by gas passage 27 can be properly detected tominimize the release of the blow-by gas into the atmosphere and theadverse effect on the internal combustion engine 1.

[0048] Moreover, the blow-by gas passage abnormality detecting systemfor an internal combustion engine according to the present embodimentsets the decision value A corresponding to the ISC opening as thecontrol amount for each engine RPM. As a result, the abnormalitydecision of the blow-by gas passage 27 is accurately made even if theengine RPM is changed by the load fluctuations.

[0049] In the present embodiment, moreover, whether or not theabnormality exists is decided at the idle time. If the engine RPM of theinternal combustion engine 1 is added to the operational condition sothat the decision on whether or not the abnormality exists may be madeat an idle time nor more than a predetermined RPM (e.g., 1,000 RPM), anerroneous determination is prevented in a transient state such as at adeceleration to improve the accuracy of the abnormality decision.

[0050] <Embodiment 2>

[0051] In the second embodiment shown in FIG. 5, the entire constructionof the blow-by gas passage abnormality detecting system for an internalcombustion engine according to the present embodiment is identical tothat of the schematic diagram of FIG. 1 in the aforementioned firstembodiment.

[0052] In FIG. 5, at step S301, it is decided whether or not the runningstate of the internal combustion engine 1 is idle. When the decision ofstep S301 is NO indicating that the running state is other than the idletime, the present routine is ended without any operation. On the otherhand, when the decision of step S301 is YES, that is, when the throttleopening of the throttle valve 13 is less than a predetermined value toindicate that the running state is idle, the routine advances to stepS302, at which it is decided whether or not the prevailing A/F (air/fuelratio) F/B (feedback) amount on the basis of the output coming from theoxygen concentration sensor 25 are more than a decision value B. Thisdecision value B is set to the A/F F/B amount for the idle time, ascannot occur if the blow-by gas passage 27 is normal.

[0053] The setting of this decision value B is based on the fact thatthe A/F F/B amount is not highly fluctuated, when the blow-by gaspassage 27 is normal, whereas an unlikely excess intake supply is madefrom the side of the blow-by gas passage 27, when this passage 27 causesthe leakage or disengagement, so that the A/F F/B amount is highlyfluctuated. Here, the decision value B may be so set at each engine RPMas to rise according to the rise in the engine RPM, as illustrated inthe graph of FIG. 6. In the abnormality decision of the presentembodiment, moreover, the A/F F/B amount is compared with the decisionvalue B, but similar effects can be expected even if either the learnedA/F F/B value or the sum of the A/F F/B value and the learned A/F F/Bvalue are compared with the decision value.

[0054] When the decision of step S302 is NO indicating that the A/F F/Bamount is no more than the decision value B, the routine advances tostep S303, at which it is ended by determining the normality in theblow-by gas passage 27 from the head cover 6 to the surge tank 14. Onthe other hand, when the decision of step S302 is YES indicating thatthe A/F F/B amount is more than the decision value B, it is deemed thatthe blow-by gas passage 27 has the leakage or disengagement, the routineadvances to step S304, at which an abnormality is determined in theleakage or disengagement of the blow-by gas passage 27 from the headcover 6 to the surge tank 14. Then, the routine advances to step S305,at which it is ended by executing an after-processing of the abnormalitydecision similar to that of FIG. 4. Thus, it is possible to properlydetect the abnormality (i.e., the leakage or the disengagement) of theblow-by gas passage 27 from the head cover 6 to the surge tank 14.

[0055] Thus, the blow-by gas passage abnormality detecting system for aninternal combustion engine according to the present embodiment measuresthe A/F F/B amount of the internal combustion engine 1 and decides thatthe blow-by gas passage 27 has the leakage or disengagement, by theabnormality decision achieved by the ECU 40, when the A/F F/B amount ishigher than the decision value B or the predetermined value. As aresult, it can be decided from the more intake flow coming from theothers than the ordinary one that the A/F F/B amount of the internalcombustion engine 1 in the ECU 40 for achieving the abnormality decisionis higher than a predetermined value, and it can be deemed that theleakage or disengagement occurs in the blow-by gas passage 27. By thustaking into consideration the A/F F/B amount of the internal combustionengine 1, the abnormality of the blow-by gas passage 27 can be properlydetected to minimize the release of the blow-by gas into the atmosphereand the adverse effect on the internal combustion engine 1.

[0056] Moreover, the blow-by gas passage abnormality detecting systemfor an internal combustion engine according to the present embodimentsets the decision value B corresponding to the A/F F/B for each engineRPM. As a result, the abnormality decision of the blow-by gas passage 27is accurately made even if the engine RPM is changed by the loadfluctuations.

[0057] <Embodiment 3>

[0058] In the third embodiment shown in FIG. 7, the entire constructionof the blow-by gas passage abnormality detecting system for an internalcombustion engine according to the present embodiment is modified fromthe schematic diagram of FIG. 1 of the first embodiment by arrangingadditionally an intake pressure sensor for detecting the intake pressurein the surge tank leading from the intake passage 12.

[0059] In FIG. 7, at step S401, it is decided whether or not the runningstate of the internal combustion engine 1 is idle. When the decision ofstep S401 is NO indicating that the running state is other than the idletime, the present routine is ended without any operation. On the otherhand, when the decision of step S401 is YES, that is, when the throttleopening of the throttle valve 13 is less than a predetermined value toindicate that the running state is idle, the routine advances to stepS402, at which it is decided whether or not the prevailing intakepressure detected by the intake pressure sensor is more than a decisionvalue C. This decision value C is set to the intake pressure for theidle time, as cannot occur if the blow-by gas passage 27 is normal.

[0060] The setting of this decision value C is based on the fact thatthe intake pressure is within a predetermined negative pressure range,when the blow-by gas passage 27 is normal, whereas a positive pressurenear the atmospheric level from the side of the blow-by gas passage 27is supplied as the suction amount, when this passage 27 causes theleakage or disengagement, so that the intake pressure highly exceeds apredetermined negative pressure range. Here, the decision value C may beso set at each engine RPM as to rise according to the rise in the engineRPM, as illustrated in the graph of FIG. 8.

[0061] When the decision of step 402 is NO indicating that the intakepressure is no more than the decision value C, the routine advances tostep 403, at which it is ended by determining the normality in theblow-by gas passage 27 from the head cover 6 to the surge tank 14. Onthe other hand, when the decision of step 402 is YES to indicate thatthe intake pressure is more than the decision value C, it is deemed thatthe blow-by gas passage 27 has the leakage or disengagement, the routineadvances to step 404, at which an abnormality is determined in theleakage or disengagement of the blow-by gas passage 27 from the headcover 6 to the surge tank 14. Then, the routine advances to step 405, atwhich it is ended by executing an after-processing of the abnormalitydecision similar to that of FIG. 4. Thus, it is possible to properlydetect the abnormality (i.e., the leakage or the disengagement) of theblow-by gas passage 27 from the head cover 6 to the surge tank 14.

[0062] Thus, the blow-by gas passage abnormality detecting system for aninternal combustion engine according to the present embodiment decidesthat the blow-by gas passage 27 causes the leakage or disengagement, bythe abnormality decision achieved by the ECU 40, when the load amountrepresented by the intake pressure detected by the pressure sensor ishigher than the decision value C or the predetermined value. By thesimple and inexpensive construction for detecting the intake pressurecorresponding to the load amount by using the intake pressure sensor, itcan be decided that the intake pressure is influenced by a pressure(e.g., the atmospheric pressure) coming from others if it is higher thana predetermined level, and it can be deemed that the leakage ordisengagement occurs in the blow-by gas passage 27. By thus taking intoconsideration the intake pressure corresponding to the load amount ofthe internal combustion engine 1, the abnormality of the blow-by gaspassage 27 can be properly detected to minimize the release of theblow-by gas into the atmosphere and the adverse effect on the internalcombustion engine 1.

[0063] Moreover, the blow-by gas passage abnormality detecting systemfor an internal combustion engine according to the present embodimentsets the decision value C corresponding to the intake pressure for eachengine RPM. As a result, the abnormality decision of the blow-by gaspassage 27 is accurately made even if the engine RPM is changed by theload fluctuations. Moreover, the accuracy of the abnormality decision isfurther improved by correcting the decision value C with the atmosphericpressure.

[0064] In the above first to third embodiments, before performing theabnormality decision of the blow-by gas passage 27, anormality/abnormality deciding routine of the intake system device orthe like can be added, as shown in FIG. 9. Specifically, the decisionsof the normality/abnormality are executed on the air flow meter 33 atstep S501, on the ISC valve 20 at step S502, on the (not-shown) throttlesensor for detecting the throttle opening of the throttle valve 13 atstep S503, on the (not-shown) atmospheric sensor for detecting theatmospheric pressure at step S504. When all these devices are normal,the diagnosis for executing the abnormality decision of the blow-by gaspassage 27 is allowed at step S505. When at least one device isabnormal, the diagnosis is inhibited at step S506. Then, the presentroutine is ended. This makes it possible to decide the abnormality ofthe blow-by gas passage 27 highly accurately.

[0065] Moreover, the foregoing embodiments are constructed by premisingthe internal combustion engine (i.e., the so-called “L-J engine”) inwhich the fuel injection is controlled by measuring the intake amount ofair flow into the intake passage and which is equipped with the air flowmeter 33. The application of the invention should not be limited theretobut may employ the intake pressure, as detected in the (not-shown)intake pressure sensor, for the abnormality decision of the blow-by gaspassage 27 in the internal combustion engine (i.e., the “D-J engine”) inwhich the fuel injection is controlled by measuring the pressure in theintake passage.

[0066] In the foregoing embodiments, moreover, there can be added anormality/abnormality deciding routine of the combustion system deviceor the like, as shown in FIG. 10A. The normality/abnormality decisionsare executed on the injector 16 at step S601, on an (not-shown) ignitioncoil at step S602, and on a (not-shown) pressure regulator at step S603.When all these devices are normal, the diagnosis for executing theabnormality decision of the blow-by gas passage 27 is allowed at stepS604. When at least one device is abnormal, the diagnosis is inhibitedat step S605. Then, the present routine is ended. This makes it possibleto improve the abnormality decision of the blow-by gas passage 27.

[0067] Further, in the foregoing embodiments, there can be added adiagnosis allowing/inhibiting routine at an electric load fluctuatingtime, as shown in FIG. 10B. Specifically, when it is decided at stepS701 that the electric load is fluctuated from ON to OFF or from OFF toON, a timer T is set with an initial value at step S702. When noelectric load fluctuation is at step S701, the value of timer T isdecremented at step S703. It is decided at step S704 whether or not thetimer T is decremented to zero. When the timer value is zero, thediagnosis to execute the abnormality decision of the blow-by gas passage27 is allowed at step S705. When the timer value is not zero so that apredetermined time has not elapsed from the electric load fluctuation,the diagnosis is inhibited at step S706. Then, the present routine isended. This makes it possible to improve the abnormality decision of theblow-by gas passage 27.

[0068] Moreover, the foregoing embodiments employ the PCV valve 28 whichis arranged midway of the blow-by gas passage 27 and acts as the flowrate regulating valve to be driven by the negative pressure. It may bean electromagnetically driven valve, the flow rate of which can becontrolled from the side of the ECU 40. In this modification, thedecision value in the abnormality decision of the blow-by gas passage 27may be made variable by the controlled flow rate.

[0069] Thus, in the blow-by gas passage abnormality detecting system foran internal combustion engine, the abnormality decision of the blow-bygas passage 27 by the ECU 40 is executed when the various devices of theintake system of the internal combustion engine 1 including the air flowmeter 33, the ISC valve 20, the throttle sensor and the atmosphericsensor are normal. As a result, the abnormality of the blow-by gaspassage 27 is properly detected.

[0070] Moreover, the blow-by gas passage abnormality detecting systemfor an internal combustion engine executes the abnormality decision ofthe blow-by gas passage 27 by the ECU 40 when the various devicesrelating to the combustion of the internal combustion engine 1 includingthe injector 16, the ignition coil and the pressure regulator arenormal. As a result, the abnormality of the blow-by gas passage 27 isproperly detected.

[0071] <Embodiment 4>

[0072] In a blow-by gas passage abnormality detecting system for aninternal combustion engine according to the fourth embodiment of theinvention shown in FIG. 11, in addition to the foregoing embodiments, anintake pressure sensor 31 for detecting the intake pressure in theintake passage 12 downstream of the throttle valve 13 is arranged in thesurge tank 14, and a gas pressure sensor 32 for detecting the blow-bygas pressure is arranged in the blow-by gas passage 27 connecting thePCV valve 28 and the surge tank 14. These sensors 31 and 32 are alsoconnected to the ECU 40.

[0073] The ECU 40 is programmed to perform blow-by gas passageabnormality detecting processing shown in FIG. 12 by the use of anabnormality decision table shown in FIG. 13 which shows decision valuesX, Y and Z of a negative pressure, as determined in advance byexperiments, against a blow-by gas pressure level PBG which is detectedby the gas pressure sensor 32 arranged between the PCV valve 28 arrangedmidway of the blow-by gas passage 27 and the surge tank 14 downstream ofthe former.

[0074] In this embodiment, when the blow-by gas passage 27 shown in FIG.11 clogs at point a (inlet side of the passage 27), the inside of theblow-by gas passage 27 takes an intake pressure substantially equal tothat in the surge tank 14 so that the blow-by gas pressure level PBG bythe gas pressure sensor 32 shifts to the side of a larger negativepressure than at the normal time (i.e., to the side of a larger negativepressure than the decision value X). When the blow-by gas passage 27leaks at point a (as exemplified by the disengagement of the hose of thepassage), the blow-by gas pressure level PBG detected by the gaspressure sensor 32 approaches the atmospheric pressure but slightly theside of the negative pressure (at the side where the negative pressureis smaller than the decision value Y but higher than the decision valueZ) because of the pressure loss in the midway PCV valve 28. When theblow-by gas passage 27 leaks or clogs at point b, the blow-by gaspressure level PBG by the gas pressure sensor 32 is not influenced bythe intake pressure in the surge tank 14 so that it takes asubstantially atmospheric pressure (at the side where the negativepressure is smaller than the decision value Z). Here, as the negativepressure of the blow-by gas pressure PBG rises in a manner to correspondto the load fluctuation of the internal combustion engine 1, as shown inthe graph of FIG. 14, the decision values X and Y of the negativepressure, as illustrated in FIG. 13, are shifted to the larger side. Theparameters of this load fluctuation are exemplified by the engine RPM,the engine coolant temperature, the intake air temperature, the ignitiontiming and so on of the internal combustion engine 1.

[0075] In FIG. 12, it is decided at step S111 whether or not the runningstate of the internal combustion engine 1 is idle. When the decision ofstep Sill is NO indicating that the running state is not idle, thepresent routine is ended without any further operation. On the otherhand, when the decision of step Sill is YES indicating that the runningstate is idle, the routine advances to step S112, at which it is decidedwhether or not the blow-by gas pressure level PBG from the gas pressuresensor 32 is larger than the negative pressure of the decision value X,as illustrated in FIG. 13. When the decision of step S112 is NOindicating that the blow-by gas pressure level PBG is smaller than thenegative pressure of the decision value X, the routine advances to stepS113, at which it is decided whether or not the blow-by gas pressurelevel PBG is smaller than the negative pressure of the decision value Y,as illustrated in FIG. 13. When the decision of step S113 is NOindicating that the blow-by gas pressure level PBG is larger than thenegative pressure of the decision value Y, that is, when the blow-by gaspressure level PBG is smaller than the negative pressure of the decisionvalue X and larger than the negative pressure of the decision value Y,the routine advances to step S114, at which a normality of the blow-bygas passage 27 from the head cover 6 to the surge tank 14 is decided toend the present routine.

[0076] On the other hand, when the decision of step S113 is YESindicating that the blow-by gas pressure level PBG is at a smaller sidethan the negative pressure of the decision value Y, the routine advancesto step S115, at which it is decided whether or not the blow-by gaspressure level PBG is at a smaller side than the negative pressure ofthe decision value Z which is set at a smaller side than the negativepressure of the decision value Y, as illustrated in FIG. 13. When thedecision of step S115 is NO indicating that the blow-by gas pressurelevel PBG is at a larger side than the negative pressure of the decisionvalue Z, that is, when the blow-by gas pressure level PBG is smallerthan the negative pressure of the decision value Y and larger than thenegative pressure of the decision value Z, as illustrated in FIG. 13,the routine advances to step S116, at which an abnormality of leakage atpoint a (as should be referred to FIG. 11) of the blow-by gas passage 27between the PCV valve 28 arranged midway of the blow-by gas passage 27and the head cover 6 upstream of the former is decided.

[0077] On the other hand, when the decision of step S115 is YESindicating that the blow-by gas pressure level PBG is at a smaller sidethan the negative pressure of the decision value Z, the routine advancesto step S117, at which an abnormality of clogging or leaking at point b(as should be referred to FIG. 11) of the blow-by gas passage 27 betweenthe PCV valve 28 arranged midway of the blow-by gas passage 27 and thesurge tank 14 downstream of the former is decided. On the other hand,when the decision condition of step S112 is YES indicating that theblow-by gas pressure level PBG is at a larger side than the negativepressure of the decision value X, the routine advances to step S118, atwhich a clogging at point a (as should be referred to FIG. 11) of theblow-by gas passage 27 between the PCV valve 28 arranged midway of theblow-by gas passage 27 and the head cover 6 upstream of the former isdecided. When an abnormality is decided at step S116, step S117 or stepS118, the routine advances to step S119, at which the after-processingof the abnormality decision is executed, and the present routine isended. Thus, it is possible to properly detect the abnormal state (e.g.,leakage or clogging) at the individual points of the blow-by gas passage27 from the head cover 6 to the surge tank 14.

[0078] In this embodiment, as shown in FIG. 11, the gas pressure sensor32 for detecting the negative pressure in the blow-by gas passage 27 isarranged between the PCV valve 28 and the surge tank 14 downstream ofthe former. As modified in FIG. 15, however, the gas pressure sensor 32can be arranged between the PCV valve 28 and the head cover 6 upstreamof the former. In this modification, too, the decision ofnormality/abnormality may be made in the similar manner as in the fourthembodiment, based upon the negative pressure state detected by the gaspressure sensor 32.

[0079] The processing procedure after the abnormality decision of stepS119 of FIG. 12 in the ECU 40 is shown in FIG. 16 which is similar tothe after-abnormality processing of the first to third embodiments shownin FIG. 4. Once it is decided that an abnormality (e.g., leakage orclogging) occurs in the blow-by gas passage 27, the following variousdetections and controls are inhibited because the reliabilities will notbe maintained.

[0080] At first step S211, there is inhibited the trouble detection ofthe intake pressure sensor 31 for detecting the intake pressure in thesurge tank 14 downstream in the intake passage 12 downstream of thethrottle valve 13. At next step S212, the trouble detection of the ISCvalve 20 is inhibited. At next step S213, the trouble detection of theISC system is inhibited. At next step S214, the trouble detection of theEGR is inhibited. At next step S215, the trouble detection of the fuelline (for controlling the fuel injection time or the like) system isinhibited. Next, the routine advances to step S216, at which the misfiredetection is inhibited. At next step S217, the F/B (feedback) control ofthe idle RPM in the ISC system is inhibited (or to lower its controlgain). At next step S218, the A/F (air/fuel ratio) learning control isinhibited. At nest step S219, the EGR control is inhibited (or to lowerits control gain). At next step S220, the control of the purge of theevaporated fuel is inhibited (or to lower its control gain). Then, thepresent routine is ended.

[0081] Thus, in the blow-by gas passage abnormality detecting system foran internal combustion engine of the fourth embodiment, the abnormalityof the blow-by gas passage 27 is decided by the gas pressure sensor 32for detecting the blow-by gas pressure level PBG in the blow-by gaspassage 27 to return the blow-by gas produced in the internal combustionengine 1 to the side of the intake passage 12 of the internal combustionengine 1 and by the ECU 40 on the basis of the blow-by gas pressurelevel PBG detected by the gas pressure sensor 32. By the simple andinexpensive construction for detecting the blow-by gas pressure levelPBG in the blow-by gas passage 27, therefore, the abnormality of theblow-by gas passage 27 is properly detected. This makes it possible tominimize the release of the blow-by gas into the atmosphere and theadverse effect on the internal combustion engine 1.

[0082] <Embodiment 5>

[0083] In the fifth embodiment also, the intake pressure sensor 31 isarranged in the surge tank 14 in addition to the gas pressure sensor 32as used in the fourth embodiment and arranged midway of the blow-by gaspassage 27, so that a negative pressure difference ΔP or the differencebetween the intake pressure level from the intake pressure sensor 31 andthe blow-by gas pressure level from the gas pressure sensor 32 is usedfor detecting the blow-by gas passage abnormality.

[0084] This detection processing is shown in FIG. 17 and uses anabnormality decision table of decision values D, E and F shown in FIG.18. Those values are determined in advance from experiments or the likeand correspond to the negative pressure difference ΔP between the intakepressure level from the intake pressure sensor 31 and the blow-by gaspressure level from the gas pressure sensor 32.

[0085] In this embodiment, similarly to the fourth embodiment (FIG. 11),when the blow-by gas passage 27 clogs at point a, the inside of theblow-by gas passage 27 approaches the intake pressure in the surge tank14 so that the negative pressure difference ΔP between the intakepressure level by the intake pressure sensor 31 and the blow-by gaspressure level by the gas pressure sensor 32 shifts to the negative (−)side (i.e., to the negative side from the decision value D) from thenormal one. On the other hand, when the blow-by gas passage 27 leaks (tocause an disengagement of the passage, for example) at point a, theinside of the blow-by gas passage 27 approaches the atmospheric pressureso that the negative pressure difference ΔP shifts to the positive (+)side (i.e., closer to the positive side than the decision value E and tothe negative side than the decision value F) from the normal one becauseof the pressure loss in the midway PCV valve 28. Moreover, when theblow-by gas passage 27 leaks or clogs at point b, the blow-by gaspressure level by the gas pressure sensor 32 takes a substantiallyatmospheric level without being influenced by the intake pressure levelin the surge tank 14, so that the negative pressure difference ΔPbetween the intake pressure level by the intake pressure sensor 31 andthe blow-by gas pressure level by the gas pressure sensor 32 highlyshifts to the positive side (i.e., closer to the positive side than thedecision value F).

[0086] In FIG. 17, it is decided at step S311 whether or not the runningstate of the internal combustion engine 1 is idle. When the decision ofstep S311 is NO indicating that the running condition is not idle, thepresent routine is ended without any operation. On the other hand, whenthe decision of step S311 is YES indicating that the running state isidle, the routine advances to step S312, at which it is decided whetheror not the negative pressure difference ΔP or the difference between theintake pressure level from the intake pressure sensor 31 and the blow-bygas pressure level from the gas pressure sensor 32 is less than thedecision value D and highly at the negative side. When the decision ofstep S312 is NO indicating the negative pressure difference ΔP is at thepositive side no less than the decision value D, the routine advances tostep S313, at which it is decided whether or not the negative pressuredifference ΔP is over the decision value E and at the positive side, asillustrated in FIG. 18. When the decision of step S313 is NO indicatingthat the negative pressure difference ΔP is no more than the decisionvalue E, that is, when the negative pressure difference ΔP is no lessthan the decision value D and no more than the decision value E, asillustrated in FIG. 18, the routine advances to step S314, at which thenormality of the blow-by gas passage 27 from the head cover 6 to thesurge tank 14 is decided to end the present routine.

[0087] On the other hand, when the decision of step S313 is YESindicating that the negative pressure difference ΔP is over the decisionvalue E and at the positive side, the routine advances to step S315, atwhich it is decided whether or not the negative pressure difference ΔPis over and highly at the positive side of the decision value F set atthe positive side of the decision value E, as illustrated in FIG. 18.When the decision of step S315 is NO indicating that the negativepressure difference ΔP is below the decision value F and at the negativeside, when the negative pressure difference ΔP is over the decisionvalue E and no more than the decision value F, as illustrated in FIG.18, the routine advances to step S316, at which the abnormality(leakage) at point a of the blow-by gas passage 27 between the PCV valve28 arranged midway of the blow-by gas passage 27 and the head cover 6upstream of the former is decided.

[0088] On the other hand, when the decision of step S315 is YESindicating that the negative pressure difference ΔP is over the decisionvalue F and at the positive side, the routine advances to step S317, atwhich an abnormality (leakage or clogging) at point b of the blow-by gaspassage 27 between the PCV valve 28 arranged midway of the blow-by gaspassage 27 and the surge tank 14 downstream of the former is decided. Onthe other hand, when the decision of step S312 is YES indicating thatthe negative pressure difference ΔP is at the negative side no less thanthe decision value D, the routine advances to step S318, at which anabnormality (clogging) at point a of the blow-by gas passage 27 betweenthe PCV valve 28 arranged midway of the blow-by gas passage 27 and thehead cover 6 upstream of the former is decide. When the abnormality isdecided at step S316, at step S317 or at step S318, the routine advancesto step S319, at which the after-processing of the abnormality decisionsimilar to that shown in FIG. 16 is executed to end the present routine.Thus, it is possible to properly detect the abnormal state (leakage orclogging) at each point of the blow-by gas passage 27 from the headcover 6 to the surge tank 14.

[0089] Thus in the blow-by gas passage abnormality detecting system foran internal combustion engine according to the fifth embodiment, anabnormality of the blow-by gas passage 27 is decided in the ECU 40 bythe gas pressure sensor 32 for detecting the pressure in the blow-by gaspassage 27 between the PCV valve 28 acting as the flow rate controlvalve for controlling the flow rate to pass through the blow-by gaspassage 27 to return the blow-by gas produced in the internal combustionengine 1 to the side of the intake passage 12 of the internal combustionengine 1 and the side of the intake passage 12 downstream of the PCVvalve 28, and by the intake pressure sensor 31 for detecting thepressure in the intake passage 12, such that the differential pressureΔP between the pressure detected by the gas pressure sensor 32 and thepressure detected by the intake pressure sensor 31 is compared with thedecision values D, E and F at the predetermined values. By the simpleand inexpensive construction for detecting the differential pressure ΔPbetween the pressure in the blow-by gas passage 27 downstream of the PCVvalve 28 and the pressure in the intake passage 12, therefore, anabnormality (leakage or clogging) of the blow-by gas passage 27 upstreamor downstream of the PCV valve 28 can be properly detected in view ofthe differential pressure ΔP over the decision values D, E and F therebyto minimize the release of the blow-by gas into the atmosphere and theadverse effect on the internal combustion engine 1. Here, theabnormality of the blow-by gas passage 27 can likewise be decided, too,by arranging the gas pressure sensor 32 between the upstream of the PCVvalve 28 and the side of the internal combustion engine 1 and bydetecting the pressure in the blow-by gas passage 27 inbetween.

[0090] In the fourth and fifth embodiments, the abnormality decision ofthe blow-by gas passage 27 is executed only at the idle time. However,it should not be limited thereto, but a similar detection can beexecuted in operations other than the idle time if the decision valuesare set to correspond to the running state.

[0091] Before the abnormality decision of the blow-by gas passage 27 inthe fourth and fifth embodiments, moreover, there can be added anormality/abnormality deciding routine of the intake system device andso on, as shown in FIG. 19. The normality/abnormality decisions areexecuted in the intake pressure sensor 31 at step S411, in the ISC valve20 at step S412, in the throttle sensor for detecting the throttleopening of the throttle valve 13 at step S413, and in the atmosphericsensor for detecting the atmospheric pressure at step S414. When all thedevices are normal, the diagnosis for executing the abnormality decisionof the blow-by gas passage 27 is allowed at step S415. When any of thedevices is abnormal, the diagnosis is inhibited at step S416. Then, thepresent routine is ended. This makes it possible to perform theabnormality decision of the blow-by gas passage 27 highly accurately.

[0092] Moreover, the fourth and fifth embodiments are constructed bypremising the internal combustion engine (i.e., the so-called “D-Jengine”) in which the fuel injection is controlled by measuring thepressure into the intake passage and which is equipped with the intakepressure sensor 31. It may employ the intake flow, as detected by an airflow meter and converted into the intake pressure at that time, for theabnormality decision of the blow-by gas passage 27 in the internalcombustion engine the “L-J engine”) in which the fuel injection iscontrolled by measuring the intake flow into the intake passage.

[0093] Moreover, there can be added to the fourth and fifth embodimentsa normality/abnormality deciding routine of the combustion system deviceor the like, as shown in FIGS. 10A and 10B.

[0094] The present invention having been described with reference to thefirst to fifth embodiments may be modified or altered further withoutdeparting from the spirit and scope of the invention.

What is claimed is:
 1. A blow-by gas passage abnormality detectingsystem for an internal combustion engine, comprising: idle statedetecting means for detecting that a running state of said internalcombustion engine is idle; and leakage detecting means for detecting aleakage of a blow-by gas coming from a blow-by gas passage for returningthe blow-by gas produced in said internal combustion engine to an intakepassage, on the basis of a parameter varying with a change in the airflow to be sucked into said internal combustion engine, when the idlestate is detected by said idle state detecting means.
 2. A blow-by gaspassage abnormality detecting system according to claim 1 , furthercomprising: control operating means for operating the control of an airflow regulator valve for regulating the air flow to be fed to saidinternal combustion engine, so that an engine RPM at an idle time ofsaid internal combustion engine may be a predetermined RPM, wherein saidleakage detecting means detects leakage of the blow-by gas on the basisof the control operated by said control operating means.
 3. A blow-bygas passage abnormality detecting system according to claim 1 , whereinsaid leakage detecting means detects leakage of the blow-by gas on thebasis of an air/fuel ratio of said internal combustion engine.
 4. Ablow-by gas passage abnormality detecting system according to claim 1 ,wherein said leakage detecting means detects leakage of the blow-by gason the basis of an intake pressure of said internal combustion engine.5. A blow-by gas passage abnormality detecting system according to claim1 , wherein said leakage detecting means detects leakage of the blow-bygas by comparing said parameter with a predetermined value and includesmeans for setting said predetermined value in accordance with an engineRPM.
 6. A blow-by gas passage abnormality detecting system according toclaim 1 , wherein said leakage detecting means executes leakagedetection when various devices relating to at least one of an engineintake system and combustion of said internal combustion engine arenormal.
 7. A blow-by gas passage abnormality detecting systemcomprising: a flow control valve for controlling a flow to pass througha blow-by gas passage to return a blow-by gas produced by an internalcombustion engine to an intake passage of said internal combustionengine; downstream pressure detecting means arranged in said blow-by gaspassage for detecting a pressure downstream of said flow control valve;and abnormality deciding means for deciding that an abnormality occursin said blow-by gas passage when the pressure detected by saiddownstream pressure detecting means is lower than a first predeterminedlevel.
 8. A blow-by gas passage abnormality detecting system accordingto claim 7 , wherein said abnormality deciding means includes means fordeciding that an abnormality occurs in said blow-by gas passage, whenthe pressure detected by said downstream pressure detecting means ishigher than a second predetermined level higher than said firstpredetermined level.
 9. A blow-by gas passage abnormality detectingsystem according to claim 8 , wherein said first and secondpredetermined levels are set with reference to the pressure detected bysaid pressure detecting means, when said blow-by gas passage is normal.10. A blow-by gas passage abnormality detecting system according toclaim 7 , wherein said abnormality deciding means executes anabnormality decision when various devices of at least one of an engineintake system and combustion system of said internal combustion engineare normal.
 11. A blow-by gas passage abnormality detecting systemaccording to claim 7 , wherein said abnormality deciding means executesan abnormality decision when a running state of said internal combustionengine is stable.
 12. A blow-by gas passage abnormality detecting systemaccording to claim 7 , further comprising: inhibition means forinhibiting trouble detections of various devices of at least one of anengine intake system and combustion system of said internal combustionengine when it is decided by said abnormality decision means that anabnormality occurs in said blow-by gas passage.
 13. A blow-by gaspassage abnormality detecting system according to claim 7 , furthercomprising: inhibition means for inhibiting detection of troubles in acontrol system of said internal combustion engine when it is decided bysaid abnormality decision means that an abnormality occurs in saidblow-by gas passage.
 14. A blow-by gas passage abnormality detectingsystem according to claim 7 , further comprising: control inhibitionmeans for inhibiting a portion of controls of said internal combustionengine when it is decided by said abnormality decision means that anabnormality occurs in said blow-by gas passage.
 15. A blow-by gaspassage abnormality detecting system according to claim 14 , whereinsaid control inhibition means includes means for inhibiting an exhaustgas recirculation control to recirculate exhaust gas of said internalcombustion engine to said intake passage.
 16. A blow-by gas passageabnormality detecting system according to claim 14 , wherein saidcontrol inhibition means includes means for inhibiting a purge controlto purge an evaporated gas produced in a fuel tank to said intakepassage.
 17. A blow-by gas passage abnormality detecting systemcomprising: a flow control valve for controlling a flow to pass througha blow-by gas passage to return blow-by gas produced by an internalcombustion engine to an intake passage of said internal combustionengine; upstream pressure detecting means arranged in said blow-by gaspassage for detecting a pressure upstream of said flow control valve;and abnormality deciding means for deciding that an abnormality occursin said blow-by gas passage when the pressure detected by said upstreampressure detecting means is lower than a first predetermined level. 18.A blow-by gas passage abnormality detecting system according to claim 17, wherein said abnormality deciding means includes means for decidingthat the abnormality occurs in said blow-by gas passage, when thepressure detected by said upstream pressure detecting means is higherthan a second predetermined level higher than said first predeterminedlevel.
 19. A blow-by gas passage abnormality detecting systemcomprising: a flow control valve for controlling a flow to pass througha blow-by gas passage to return blow-by gas produced by an internalcombustion engine to an intake passage of said internal combustionengine; upstream pressure detecting means arranged in said blow-by gaspassage for detecting a pressure upstream of said flow control valve;intake pressure detecting means for detecting a pressure in said intakepassage; and abnormality deciding means for deciding that an abnormalityoccurs in said blow-by gas passage, on the basis of the pressuredetected by said upstream pressure detecting means and the pressuredetected by said intake pressure detecting means.
 20. A blow-by gaspassage abnormality detecting system according to claim 19 , whereinsaid abnormality deciding means includes means for deciding anabnormality by comparing a differential pressure between the pressuredetected by said upstream pressure detecting means and the pressuredetected by said intake pressure detecting means, with a predeterminedlevel, and wherein said predetermined level is set on the basis of adifferential pressure between the pressure detected by said upstreampressure detecting means and the pressure detected by said intakepressure detecting means when said blow-by gas passage is normal.
 21. Ablow-by gas passage abnormality detecting system comprising: a flowcontrol valve for controlling a flow to pass through a blow-by gaspassage to return blow-by gas produced by an internal combustion engineto an intake passage of said internal combustion engine; downstreampressure detecting means arranged in said blow-by gas passage fordetecting a pressure downstream of said flow control valve; intakepressure detecting means for detecting a pressure in said intakepassage; abnormality deciding means for deciding that an abnormalityoccurs in said blow-by gas passage, on the basis of the pressuredetected by said downstream pressure detecting means and the pressuredetected by said intake pressure detecting means; and control inhibitionmeans for inhibiting a portion of controls of said internal combustionengine when it is decided by said abnormality deciding means that anabnormality occurs in said blow-by gas passage.
 22. A blow-by gaspassage abnormality detecting system according to claim 21 , whereinsaid control inhibition means includes means for invalidating an exhaustgas recirculation control to recirculate exhaust gas of said internalcombustion engine to said intake passage.
 23. A blow-by gas passageabnormality detecting system according to claim 21 , wherein saidcontrol inhibition means includes means for inhibiting a purge controlto purge an evaporated gas produced in a fuel tank to said intakepassage.
 24. A blow-by gas passage abnormality detecting systemcomprising: abnormality deciding means for deciding an abnormality of ablow-by gas passage to return blow-by gas produced in an internalcombustion engine to an intake passage of said internal combustionengine; and decision allowing means for allowing an abnormality decisionby said abnormality deciding means when various devices of at least oneof an intake system and combustion system of said internal combustionengine are normal.
 25. A blow-by gas passage abnormality detectingsystem comprising: abnormality deciding means for deciding anabnormality of a blow-by gas passage to return a blow-by gas produced inan internal combustion engine to an intake passage of said internalcombustion engine; and decision allowing means for allowing anabnormality decision by said abnormality deciding means when the runningstate of said internal combustion engine is stable.
 26. A blow-by gaspassage abnormality detecting system comprising: abnormality decidingmeans for deciding an abnormality of a blow-by gas passage to returnblow-by gas produced in an internal combustion engine to an intakepassage of said internal combustion engine; and inhibition means forinhibiting trouble detections of various devices of at least one of anintake system and a combustion system of said internal combustion enginewhen it is decided by said abnormality deciding means that anabnormality occurs in said blow-by gas passage.
 27. A blow-by gaspassage abnormality detecting system comprising: abnormality decidingmeans for deciding an abnormality of a blow-by gas passage to returnblow-by gas produced in an internal combustion engine to an intakepassage of said internal combustion engine; and inhibition means forinhibiting trouble detections of a control system of said internalcombustion engine when it is decided by said abnormality deciding meansthat an abnormality occurs in said blow-by gas passage.
 28. A blow-bygas passage abnormality detecting system comprising: abnormalitydeciding means for deciding an abnormality of a blow-by gas passage toreturn a blow-by gas produced in an internal combustion engine to anintake passage of said internal combustion engine; and inhibition meansfor inhibiting a portion of controls of said internal combustion enginewhen it is decided by said abnormality deciding means that anabnormality occurs in said blow-by gas passage.
 29. A blow-by gaspassage abnormality detecting system according to claim 28 , whereinsaid control inhibition means includes means for inhibiting an exhaustgas recirculation control to recirculate exhaust gas of said internalcombustion engine to said intake passage.
 30. A blow-by gas passageabnormality detecting system according to claim 28 , wherein saidcontrol inhibition means includes means for invalidating a purge controlto purge an evaporated gas produced in a fuel tank to said intakepassage.